In case this is useful to others, the line I commented out in file class.projectmanager_ui.inc.php
//self::_project_tree_leaves($filter,$parent_pm_id?$parent_pm_id : 'mains',$_pm_id ? $_pm_id : $parent_pm_id,$nodes);
Lastly, a little CSS to hide the dead tree.
#projectmanager-tree_target